feat: 预览区划选文字浮窗「添加到对话」（md 预览 + 兜底 code 查看器）

src/client/sidebar.module.css

@@ -1067,6 +1067,32 @@
   font: var(--dsw-font-xs-13);
 }
 
+/* The floating "add to conversation" button anchored above a text
+   selection in the preview/code viewers. Portalled to document.body and
+   position:fixed, so the editor's overflow clips cannot crop it; flat
+   chrome like the rest of the sidebar (hairline border, layer-2 fill,
+   hover fill — no shadow). */
+.selectionPopup {
+  position: fixed;
+  z-index: 60;
+  transform: translate(-50%, calc(-100% - 8px));
+  display: inline-flex;
+  align-items: center;
+  height: 28px;
+  padding: 0 10px;
+  border: 1px solid var(--dsw-alias-border-l1);
+  border-radius: 6px;
+  background: var(--dsw-alias-bg-layer-2);
+  color: var(--dsw-alias-label-primary);
+  font: var(--dsw-font-xxxs-strong-11);
+  white-space: nowrap;
+  cursor: pointer;
+}
+
+.selectionPopup:hover {
+  background: var(--dsw-alias-interactive-bg-hover);
+}
+
 /* Word preview: the document scrolls independently while the zoom control
    stays pinned to the bottom edge. */
 .editorDocx {